This invention relates to a switching device, and in particular to a photoconductive or other irradiation activated switching device fabricated from diamond.
The ideal high voltage switch provides a perfect current block in the ‘off’ state, able to withstand the high voltages applied without breakdown, and in the ‘on’ state has zero impedance to the current being switched, even where this current is high. All practical switching materials and devices fall short of this ideal, and an area of particular difficulty is in switches required to withstand both high voltages in the ‘off’ state and high currents in the ‘on’ state.
As the voltage rating of a system incorporating a high voltage switch increases, the switch used needs to withstand higher voltages in the off state, such that the reverse blocking capability of the switch needs to be as high as possible. Using silicon for such a switch would involve excessively large voltage blocking layers, which may not be realistically possible (especially if reverse voltages in excess of 10 kV are involved), as this will lead to large on-resistances. Whilst high blocking voltages are sustainable in suitable high quality diamond material, the problem of using such material has been in terms of a limited current capability and a relatively high impedance in the on state, these two problems being related.
The applications of such switches are considered in certain prior art references. For example, U.S. Pat. No. 6,239,514 discusses the general concept of using an optically or radiation activated switch in parallel with a mechanical switch, the optically or radiation activated switch relieving the mechanical switch from inductive arcing as the mechanical switch is opened. U.S. Pat. No. 6,140,715 describes a structure where the radiation activated switch is based on a PIN diode structure. Other prior art references consider radiation activated switches in more detail, for example, U.S. Pat. No. 6,194,699 concerns itself with the problem of shadowing, where the contact structure on the side of the device on which the radiation beam impinges shadows the important region of the contact with the intrinsic semi-conductor material to which it makes contact. It proposes a solution of providing an additional layer at this surface that permits mobility of the carriers and that may bury the contacts. U.S. Pat. No. 6,222,141 proposes an alternative solution to this problem, laterally displacing electrode structures on opposite major faces and irradiating both major faces, so that no region is in shadow from both sides.
A third area of concern addressed in the prior art is the provision of a device in which the applied voltage may remain in the same direction for both conducting and blocking states, and in which carrier injection is simplified. U.S. Pat. No. 6,204,522 thus covers the use of an intermediate layer such as SiC between the contacts and the intrinsic wide band gap material such as diamond. Such SiC layer generates carriers under lower energy radiation, and by controlling the injection of carriers enables the device to turn off under high applied voltage as the radiation source is removed. However, such devices are difficult and expensive to fabricate, being reliant on a high quality hetero-epitaxial interface between the diamond and the SiC.